This invention relates to a lightly crosslinked silane functional vinylidene chloride homopolymer or copolymer suitable for use as a film, coating, or foam, and more particularly to a polymer composition and process which is the reaction product of vinylidene chloride, an optional copolymerizable monomer, and an .alpha.,.beta. ethylenically unsaturated silane.
Vinylidene chloride polymers are known to possess desirable chemical and physical properties including resistance to ignition and combustion, toughness, insolubility in common solvents, and low vapor and gas transmission rates. Such polymers have heretofore been used in synthetic fibers, coatings, and films. However, vinylidene chloride polymers, being essentially linear polymers, are known to have poor melt strengths. Moreover, polymers of vinylidene chloride have not been readily adaptable to conventional extrusion techniques used to produce foamed cellular structures. This has been due to several factors.
Initially, polymers of vinylidene chloride tend to degrade with the evolution of hydrogen chloride at temperatures only slightly above the temperatures necessary for melt processing. Additionally, most vinylidene chloride polymers, being essentially linear in nature, have poor melt tension. A sharp decrease in melt viscosity occurs at melt processing temperatures which results in poor foam quality and many open cells. Finally, vinylidene chloride polymers are insoluble or only somewhat soluble with many conventionally used blowing agents.
Attempts have been made to produce foams of polyvinylidene chloride. For example, Suh et al, U.S. Pat. No. 3,983,080, teach forming foams of normally crystalline vinylidene chloride and copolymerizable monomers utilizing blowing agents having specified physical properties and solubility characteristics and utilizing carefully controlled temperatures. Suh et al also discuss other prior art efforts at producing vinylidene chloride polymer foams.
Hattori et al, Japanese Kokai 78/112/967, have reported producing an extruded foam utilizing a mixture of polymers containing 60% polyethylene, 20% ethylene/vinyl acetate copolymer, and 20% polyvinylidene chloride copolymerized with polyolefins or polystyrene. However, it is believed that as the vinylidene chloride content of the foam is lowered, the effects of the desirable chemical and physical properties of vinylidene chloride are lessened.
Because of the poor melt strength of vinylidene chloride polymers, after heating and melting, some prior art techniques required that the polymer be cooled to increase melt strength prior to foaming. This required careful temperature control and very narrow workable temperatures ranges. While the melt strength of such vinylidene chloride polymers may be increased by the introduction of crosslinking monomers such as diallyl ether, divinyl benzene, or diacrylates to produce higher molecular weight, crosslinked vinylidene chloride polymers, such higher molecular weight polymers would be unacceptable in an extrusion process. Such higher molecular weights would lead to the generation of shear heat in the extruder which would cause degradation of the vinylidene chloride polymer.
Accordingly, the need exists in the art for a vinylidene chloride polymeric composition and process for making it which possesses good melt strength and melt tension and which can be utilized in conventional melt processing techniques.